ial Supports in DieloswiQ <m<l Zygospvfd, 1 1 



b<irr<in<lci and (\ innn/innlix the bnichial supports, as worked out 

 by Davidson and (ilass, consist of about five volutions, and are 

 similar to those of Zi/f/ospim, except that the transverse band is 

 more posterior, since it originates near the ends of the crura. 

 This mature condition of Coelospira is seen to be a young condi- 

 tion of Atrypa (Davidson), but, as the spirals are more loosely 

 coiled and the transverse band always continuous, this genus 

 should be regarded as valid in the evolution from Zygotyira to 

 Atrypa. In mature Atrypa rrficnhris from the Upper Silurian, 

 there may be as many as sixteen volutions in each spiral cone 

 (Davidson), but more often the number is smaller. The trans- 

 verse band in this species during its young stages is continuous, 

 but in the adult condition it seems to be usually disunited in 

 the middle. This feature becomes a distinct adult character in 

 the Devonian specimens, which also have a greater number of 

 whorls in the spirals, as shown in a Chemung specimen of this 

 species in Yale University Museum, having twenty-four turns of 

 the lamella} in each spiral. 



The ontogeny and phylogeny of the species of Zygospira indi- 

 cate strongly that the Atrypidse had its origin in a form with a 

 centronelloid loop. A further natural conclusion from the same 

 evidence is that the Ancylobrachia are older and more primitive 

 than the Helicopegmata. 



EXPLANATION OF PLATE X. 

 Dielasma turgida. 



Figure 1. The centronettiform stage of the loop ; ventral view. X 6. 



2. A later stage, showing the resorption of the anterior portion 



of the loop. X 6. 

 3. Early Dielasma stage, produced by further resorption of the 



centronelloid loop. X 6. 



4. Loop and crural plates of mature specimen. X 0. 

 .5. Profile of the connecting band. X 6. 

 (>. Side view of the loop, crura, and septum. X <>. St. Louis 



group, Kentucky. 



Zygospira recurvirostra. 



Figure 7. Centronelliform stage of the loop. X 12. 

 8. Profile of same. X 12. 

 9. A later stage ; showing partial resorption of loop in front and 



greater divergence of descending branches. X 12. 

 10. The same ; profile. X 12. 



